The heating of industrial articles by microwave energy is attracting much interest these days as an alternative to conventional heating because of the improved speed and economy that can thereby be achieved. A problem which has been encountered, however, is that uniform heating of large volumes is difficult to achieve with microwaves and consequently large workpieces or groups of workpieces may not be heated uniformly. In fact, problems of lack of uniform heating are usually encountered when volumes of more than about one cubic inch (16.4 cm.sup.3) are involved. This severely limits the usefulness of microwave heating for those industrial applications in which relatively uniform heating is critical. While it is true that uniform heating of larger volumes and workpieces can be achieved if heating times are suitably prolonged (conduction and convection eventually equalize temperatures), this is obviously not an economic solution to the problem for industrial operations in which an objective is to minimize cycle times.
Even when relatively small workpieces are to be heated, it is more economical to heat a large number of components simultaneously as a relatively large batch rather than to heat them individually, thus it is important to be able to heat large volumes uniformly even with such small components.
A primary area of utilization of the invention is in the sintering of ceramic components, e.g. one or more large ceramic components, or a comparatively large number of smaller ceramic components, that are to be heated to a sintering temperature. In such cases, lack of uniformity during the heating step can result in lack of uniform density of the products. Uniformity of heating is therefore particularly important in such cases.
Similar lack of uniformity of heating has been observed when microwave energy is employed for heating ceramic components for the removal of binders, or for drying workpieces generally, or for driving physical or chemical reactions. Generally speaking, the more massive the workpiece or the assembly of workpieces, the more pronounced the non-isothermal nature of the process becomes, and this experience has in the past tended to limit the size of the workpiece assembly that it has been possible to heat with microwave energy, if quality standards are to be maintained.
In the following specification and claims, the term "workpiece assembly" has been adopted to describe either a single, bulky workpiece or starting material, or, more often, a relatively large number of smaller workpieces that together make up a bulky assembly.